In systems that combine or stitch together on the wafer a set of small subfields on the reticle, as described in S. D. Berger & J. M. Gibson, APPL. PHYS. LETTERS 57 (2) (1990) 153), there is the problem of joining up corresponding lines or pattern features on adjacent subfields. This stitching problem also exists in any lithographic system wherever fields or subfields are butted and the features that span the fields or subfields are stitched together. Imperfect stitching of features can result in short or open circuits which will cause hard failures in the integrated circuit being formed. Also, if an overlap is exposed at full strength, the extra intensity will raise portions of the resist outside the intended area above threshold, causing "blooming" in which the feature being defined has a greater size than intended. The prior art of electron-beam direct writing exposures has used a technique called grey splicing in which corresponding features are overlapped and exposed at 1/2 dose at the boundary with half the exposure of the main area.
Consequently, the art has sought a reticle for stitching that would offer an acceptable tradeoff between ensuring correct alignment and mask complexity.